1. Field of the Invention
The present invention relates to a studless pneumatic tire having excellent travel performance on roads covered partly or completely by ice and/or snow (hereinafter referred to as xe2x80x9cicy roadsxe2x80x9d). The present invention also relates to a mold used for manufacturing through a vulcanizing molding process a pneumatic tire having a block pattern, and to a pneumatic tire manufactured using the mold.
2. Description of the Related Art
Since the use of spiked tires became prohibited, various improvements have been made on tire treads so as to obtain studless tires having improved performance.
The tread of a studless tire used for travel on icy roads has a plurality (3-5) of main grooves extending in the circumferential direction and a large number of lug grooves intersecting the main grooves, thereby forming block-shaped island portions between them. In order to improve the performance of the tire on icy surfaces, many sipes are provided in the island portions, thereby increasing the cumulative length of edges (hereinafter referred to as the xe2x80x9cedge componentxe2x80x9d) of the island portions.
However, when many lateral sipes traversing the island portions are provided so as to improve driving and braking performances of the tire on icy surfaces, the overall stiffness of the blocks decreases although the edge component of the island portions increases. This increases the possibility of chipping occurring in the blocks of heavy duty tires, such as tires for trucks and buses, which are used while subject to high internal pressure and a heavy load. In tires for passenger cars, island elements, which are portions of each block finely divided by sipes, tend to bend down while cornering, resulting in decreased driving stability due to a reduced effective ground contact area.
Therefore, it is necessary to maintain the stiffness of the island portions even when many sipes are formed. For example, a tire disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2-200503 employs laterally extending sipes each having one end terminated within the island portion (i.e., only one end of each sipe is opened to a main groove) so as to maintain the stiffness of each island portion even when the density of sipes increases.
However, even when only one end of each sipe is opened to the main groove, the stiffness of each island portion decreases undesirably if many sipes are formed so as to obtain a desired edge effect.
The present inventors studied in detail the stiffness of island portions having sipes, and found that when the number of sipes is increased to obtain a desired edge effect, the stiffness at the lateral end regions of each island portion decreases more than expected as compared with the central region of the island portion, and that even when the sipes are formed such that each extends from one lateral edge of its corresponding island facing one main groove to a point near the other lateral edge thereof facing another main groove, the decrease of the stiffness cannot be completely prevented.
The present invention has been conceived in view of the above-described drawbacks, and an object of this invention is to provide a studless pneumatic tire which maintains the stiffness of island portions even when many sipes are formed, thereby providing improved driving stability and performance of the tire on icy roads while preventing island portions from being chipped.
According to a first aspect of the present invention, a studless pneumatic tire is provided which has a plurality of block-shaped island portions formed on a cylindrical tread extending between a pair of sidewalls. The island portions have laterally extending sipes and are separated by a plurality of main grooves extending in the circumferential direction as well as by many lug grooves. The sipes include first sipes and second sipes. Each first sipe extending from a first lateral end facing a first main groove traverses a first edge region of each island portion in a substantially axial direction and reaches a first interface between the first edge region and the central region of the island portion where the first sipe branches into a plurality of branch segments, thereby forming a fork-like shape. Each of these branch segments traverses the central region of the island portion and reaches a point near a second interface between a second edge region and the central region of the island portion. Each second sipe extending from a second lateral end of the island facing a second main groove traverses the second edge region and extends into the central region of the island portion in the axial direction. These first and second sipes divide the central region of the island portion into a larger number of island elements compared to the edge regions of the island portion.
In the studless pneumatic tire according to the first aspect of the present invention, the first sipe branches into a plurality of branch segments, thereby forming a fork-like shape, at the first interface between the first edge region and the central region of the island portion, and each branch segment traverses the central region and reaches a point near the second interface between the second edge region and the central region. The total projected length of the sipes in the circumferential direction increases in the central region because of the existence of the branch segments. This increases the edge component of each island portion. Further, each edge region of the island portion which is divided by the corresponding edge segment in the circumferential direction can effectively maintain the stiffness of the island portion.
In the first aspect of the present invention, when the tip of the second sipe extending from a lateral end opposite to the lateral end from which the first sipe extends is terminated at a position such that the tip of the second sipe does not intersect the first sipe, or when the second sipe has a fork-like shape similar to that of the first sipe and the tip of the second sipe is terminated at a position such that the tips of the branch segments of the second sipe do not intersect the first sipe, island elements divided by the sipes continue from a portion contacting one lug groove to a portion contacting the other lug groove in a staggered manner in the central region of the island portion. In contrast, each edge region of the island portion is divided by the edge segment of the fork-shaped sipe only, so that a decrease i n the stiffness of the island portion can be minimized.
According to a second aspect of the present invention, a studless pneumatic tire is provided which has a plurality of block-shaped island portions formed on a cylindrical tread ex tending between a pair of sidewalls. The island portions have laterally extending sipes and are separated by a plurality of main grooves extending in the circumferential direction as well as by many lug grooves. The sipes include first sipes and second sipes. Each first sipe has an extension segment and a return segment. The extension segment of the first sipe extends from a first lateral end of the island portion facing a first main groove so as to traverse a first edge region and a central region of the island portion in the axial direction. The return segment has a tip which is turned back at the interface between the central region and the second edge region or in the vicinity thereof and is terminated within the central region. Each second sipe also has an extension segment and a return segment, which are the same as those of the first sipe. By these first and second sipes, the central region of each island portion is circumferentially divided into a larger number of island elements compared to the edge regions thereof.
In the studless pneumatic tire according to the second aspect of the present invention, each block-shaped island portion in the tread has hook-shaped first and second sipes which extend in opposite directions from both lateral ends facing corresponding main grooves which extend in the circumferential direction. Therefore, the central region of the island portion is divided by both the extension segment and the return segment of each sipe. In contrast, each edge region of the island portion is divided by the extension segment of the first or second sipe only. Therefore, it is possible to form a large edge component within the central region of the island portion and to maintain a desired stiffness in each edge region of the island portion. As a result, desired driving and braking performance can be obtained, especially on icy surfaces, while preventing block chipping and decrease of driving stability, which have been considered to be problems which would occur if the desired driving and braking performance is obtained.
However, it was found in some cases that when many shapes are combined, the stiffness of each island portion does not become higher than in the case of using conventional sipes completely traversing each island portion even if the sipes having the above-described shapes are advantageous over the conventional sipes from the viewpoint of the stiffness.
The present inventors studied in detail and found that when many sipes having the above-described shapes are combined, the total projected length of portions of sipes within the central region of each island portion becomes longer than that within each edge region, so that the stiffness of the island portion within the central region thereof becomes lower than that within the edge regions. From the above, the present inventors considered that when the stiffness is increased in a region where the total projected length of the sipes is large, the overall stiffness of each island portion can be increased without decreasing the number and length of sipes.
Generally, the tread of a tire employs a two-layer structure consisting of a cap rubber layer and a base rubber layer so as to improve the heat generating performance of the tire. The present inventors found that when the above two-layer structure is modified such that in an area in which the total projected length of portions of the sipes within that area is larger than that in the remaining areas, the ratio of the volume of hard rubber to the volume of soft rubber is larger than that in the remaining areas, the stiffness of each island portion is prevented from decreasing, and that this structure makes it possible to obtain desired driving and braking performance, especially on icy surfaces, while preventing block chipping and decrease of driving stability.
In view of the forgoing, a third aspect of the present invention provides a studless pneumatic tire in which a plurality of block-shaped island portions each having sipes are provided on a cylindrical tread extending between a pair of annular sidewalls. The tread has at least two rubber layers having different hardnesses. In an area in which the total projected length of portions of the sipes within that area is larger than those in the remaining areas, the ratio of the volume of hard rubber relative to the volume of soft rubber is larger than those in the remaining areas.
Examples of the shape of each sipe include a straight shape, a wavy shape, a zigzag shape, a hook-like shape, and a fork-like shape. These shapes may be used singly or in combination.
Next, the operation of the studless pneumatic tire according to the third aspect of the present invention will be described.
In the tire according to the third aspect of the present invention, in an area in which the total projected length of portions of the sipes within that area is larger than those in the remaining areas, the ratio of the volume of hard rubber relative to the volume of soft rubber is larger than those in the remaining areas. Since a decrease in the stiffness of the island portion caused by the formation of sipes can be prevented by the existence of the hard rubber, the stiffness of the block-shaped island portion can be guaranteed.
When a soft rubber layer is disposed on the surface of the tread and a hard rubber layer is disposed radially inward of the soft rubber layer, the soft rubber layer located at the surface is well fitted to the slightly uneven surface of an icy road, resulting in increased gripping performance. Accordingly, increased traction force and braking force can be obtained.
Since the hard rubber increases the stiffness of each block-shaped island portion, bending of the island portion is suppressed, thereby guaranteeing desired driving stability. Also, the island portion is prevented from chipping.
Accordingly, it is possible to provide desired driving and braking performance, especially on icy surfaces, while solving the problems of decreased driving stability and block chipping.
According to a fourth aspect of the present invention, the studless pneumatic tire of the third aspect has a plurality of block-shaped island portions each of which has first and second sipes. The first sipe extends from a first lateral end of the island portion so as to pass through a first edge region adjacent to the first lateral end in the substantially axial direction of the tire and branches into a plurality of branch segments at a first interface between the first edge region and the central region. Each branch segment passes through the central region to a point in the vicinity of a second interface between the central region and a second edge region. The second sipe extends from a second lateral end in a substantially axial direction so as to pass through the second edge region adjacent to the second lateral end, thereby entering the central region.
The operation of the tire according to the fourth aspect of the present invention will be described.
In the studless pneumatic tire according to the fourth aspect, there are provided first and second sipes which extend from both lateral ends of each island portion in opposite directions. Therefore, the central region of each island portion is circumferentially divided into many island elements by a plurality of branch segments and the second sipe. In contrast, in each edge region, the island portion is divided into only two by the first or second sipe. Accordingly, a large edge component can be obtained in the central region of the island portion and a desired stiffness is maintained in the edge regions.
According to a fifth aspect of the present invention, the studless pneumatic tire of the third aspect has the following structure. Hook-shaped first and second sipes are provided. Each first sipe has an extension segment and a return segment. The extension segment of the first sipe extends from a first lateral end and traverses a first edge region and a central region in a substantially axial direction. One end of the return segment of the first sipe is turned back at the interface between the central region and a second edge region or in the vicinity thereof, and the other end of the return segment is terminated within the central region. Each second sipe also has an extension segment and a return segment. The extension segment of the second sipe extends from a second lateral end of the island portion traverses the second edge region and the central region of the island portion in a substantially axial direction. One end of the return segment is turned back at the interface between the central region and the first edge region or in the vicinity thereof, and the other end of the return segment is terminated within the central region.
The operation of the studless pneumatic tire according to the fifth aspect of the present invention will be described.
In the studless pneumatic tire according to the fifth aspect, there are provided first and second hook-shaped sipes which extend from both lateral ends of each island portion in opposite directions. Therefore, the central region of each island portion is circumferentially divided into many island elements by the extension segments and the return segments. In contrast, in each edge region, the island portion is divided only by the first or second sipe. Accordingly, a large edge component can be obtained in the central region of the island portion and a desired stiffness is maintained in the lateral edge regions.
As described above, in order to increase the edge component in a studless tire, thereby improving performance on icy surfaces (hereinafter referred to as xe2x80x9con-ice performancexe2x80x9d), it is effective to constitute the tread portion of the tire by block-shaped island portions divided by a plurality of main grooves extending in the circumferential direction and many lug grooves extending in a direction intersecting the main grooves, and to provide many lateral sipes in each block-shaped island portion.
Lateral sipes are generally classified into two types, i.e., both-end open type sipes (open sipes) which completely traverse island portions, and one-end open type sipes whose first ends open to a main groove and whose second ends terminate within the island portion. The former sipes have an advantage that they can increase the edge component more than the latter sipes. However, the former sipes have a disadvantage that the wear resistance against partial wear and driving stability on dry road surfaces (hereinafter referred to as xe2x80x9cdry driving stabilityxe2x80x9d) provided by the former sipes are lower than in the case of using the latter sipes because of the decreased stiffness of each island portion. On the contrary, the latter sipes have a disadvantage that the edge component provided by the latter sipes is smaller than in the case of using the former sipes. However, the latter sipes have an advantage that the wear resistance against partial wear and dry driving stability provided by the latter sipes are higher than in the case of using the latter sipes, because the stiffness of each island portion decreases due to formation of the sipes. Although the both-end open type sipes and the one-end open type sipes both have disadvantages, these disadvantages are insubstantial.
Regardless of the type of lateral sipes, blades for forming sipes must be disposed within each depression for forming a block-shaped island portion, which is provided in a mold for vulcanizing molding, when tires having such lateral sipes are manufactured. Since the sipe-forming blades are very thin plates, the stiffness thereof is usually insufficient. Therefore, there is a possibility that the blades bend during the process of vulcanizing molding.
This problem depends on the kind of tires to be manufactured. In tires for heavy loads such as tires for trucks and buses, the depth of each sipe is large, and the hardness of the tread rubber is high. Thus, a large force acts on the tread during the process of vulcanizing molding, thereby causing bending of blades.
The above-described problem also depends on the type of sipes. When one-end open type sipes are formed, only two sides among the four sides of each blade are fixed to a depression (i.e., when one-end open type sipes are formed, only one of the longitudinal ends of each blade is fixed), the supporting stiffness of each blade decreases, resulting in bending of blades.
Also, there is a need to maintain the on-ice performance of a pneumatic tire having such lateral sipes by improving the water draining performance of the tire.
Therefore, another object of the present invention is to provide a mold for vulcanizing molding which can prevent bending of blades during the process of vulcanizing molding.
Still another object of the present invention is to provide a tire which is manufactured using the mold as set forth above and which has an excellent on-ice performance.
To accomplish these objects, according to a sixth aspect of the present invention, there is provided a mold for vulcanizing molding which is used for manufacturing a pneumatic tire by vulcanizing molding. The tire has a block pattern having a plurality of block-shaped island portions, which are divided by main grooves extending in the circumferential direction of the tire and lug grooves extending in the widthwise direction of the tire. The mold has blades for forming lateral sipes, which are formed in each block-shaped island portion and extend in the widthwise direction of the tire. Each blade has at least one bent portion, and has a flask-shaped cross section at a portion corresponding to the bottom of a corresponding sipe.
According to the seventh aspect of the present invention, the portion of the blade having a flask-shaped cross section and corresponding to the bottom of a corresponding sipe is divided at the bent portion of the blade.
According to the eighth aspect of the present invention, there is provided a pneumatic tire manufactured by using the mold for vulcanizing molding according to the sixth or seventh embodiment.
According to the ninth aspect of the present invention, the pneumatic tire according to the eighth aspect has one-end open type sipes, the first ends of which are opened to a lateral end of an island portion and the second ends of which are substantially terminated within the island portion.
According to the tenth aspect of the present invention, the inner end of each lateral sipe has a flask-shaped cross section.
According to the sixth aspect of the present invention, each blade for forming a lateral sipe has at least one bent portion. Therefore, the stiffness of each blade is higher than a blade for forming a straight sipe. Accordingly, the blades do not bend during the process of vulcanizing molding.
Further, since the portion of each blade corresponding to the bottom of a lateral sipe has a flask-shaped cross section, the bottom of a lateral sipe formed by the blade also has a flask-shaped cross section, so that the on-ice performance of the tire can be maintained at a satisfactory level. In detail, when the lateral sipe has at least one bent portion so as to form a sipe having a bent shape, water draining performance decreases due to contact of the edge portions of the lateral sipe. Therefore, on icy road surfaces, there is a possibility that the frictional coefficient p of the tire decreases with a consequent decrease in the on-ice performance. In the present embodiment, a portion of each blade corresponding to the bottom of a corresponding lateral sipe is formed to have a flask-shaped cross section, thereby making the cross section of the bottom of the correspond lateral sipe in a flask-like shape, the performance of draining water from the bottom of the sipe can be improved. Thus, the frictional coefficient xcexcon icy road surfaces can be increased to maintain excellent on-ice performance.
The pneumatic tire according to the eighth aspect of the present invention manufactured by using the mold for vulcanizing molding according to the sixth or seventh aspect of the present invention can prevent bending of blades during the process of vulcanizing molding and can maintain the on-ice performance at a desired level.
Since the bottom portion of each lateral sipe has a flask-shaped cross section, stress acting on the bottom portion can be dispersed. Therefore, it is possible to prevent generation of cracks in the bottom portion of each sipe.
Further, according to the seventh aspect of the present invention depending on the sixth aspect of the present invention, the portion of each blade having a flask-shaped cross section and corresponding to the bottom of a sipe is divided at the bent portion or portions of the blade. This facilitates the manufacture of blades compared to the case where the portion having a flask-shaped cross section is not divided.
According to the ninth aspect of the present invention depending on the eighth aspect of the present invention, each lateral sipe of a pneumatic tire is of one-end open type in which one end of each sipe opens to one end of an island portion, and the other end thereof terminates within the island portion. Therefore, the stiffness of the island portion can be increased more than in the case where the lateral sipe is of a both-end open type, i.e., an open sipe. This enhances both wear resistance against partial wear and dry driving stability.
According to the tenth aspect of the present invention depending on the ninth aspect of the present invention, since not only the bottom of each blade but also one end of the blade corresponding to the inner end of a corresponding sipe has a flask-shaped cross section, generation of cracks at the other end of the lateral sipe is prevented. Therefore, generation of cracks can be prevented more effectively through the synergy between the effect of preventing cracks at the inner end of the sipe and the effect of preventing generation of cracks at the bottom portion of the sipe. Since the inner end of the hook-shaped sipe has a flask-shaped cross section, the water draining performance of the sipe at the inner end thereof can be improved.